I guess it's not popular because it lacks the appeal of a Leica lens' quality and price (oddly, a high price is sometimes a good selling tactic), and misses out on having a signature character, such as my own favorite, the Canon 35/2.0 has.
I have one, but don't like it much. The 35mm Ultron may just be the Toyota Camry of the 35mm lens world - utilitarian, generally reliable, affordable, but meh.